The present invention concerns an apparatus for washing cellulose, comprising a rotary drum with a liquid-permeable mantle surface; a stationary shell parallelling the shaft of said drum and encircling the drum in such manner that the cellulose that is being washed can pass through between the drum and the shell in the form of a layer lying against said mantle surface and through which the washing liquid can be introduced in the cellulose layer; and washing liquid input conduit which has been connected to the shell close to the washed cellulose removal point; compartments under the mantle surface of the drum in which the washing liquid that has been drained through the cellulose layer accumulates; a valve system connected to said compartments, consisting of a part rotating along with the drum and a stationary part lying against it; conduits starting at said stationary part and communicating, when the apparatus is in operation, with different compartments of the drum and connecting at different points on the shell encricling the drum so that during operation the washing liquid can be conducted several times through the cellulose layer at various points thereof and moving stepwise in the direction opposite to the direction of rotation of the drum; and a removal conduit starting at said stationary part and through which the washing liquid can be removed from the apparatus.
An apparatus of the kind described is known in the art e.g. through the Finnish Patent publications No. 56564 and No. 56865. In the types of apparatus disclosed in these references, the cellulose in conducted through between a rotating drum and a shell encircling same, in the form of a continuous web, through which the washing liquid is conducted several times by pressure produced by one single pumping operation. In a design of this kind, one is able to manage with a rather small quantity of washing liquid, and no detrimental internal movement of fibres relative to each other will occur in the web during the process.
In said types of apparatus of prior art, transfer of the washing liquid from one washing step to another, i.e., the returning of washing liquid that has been drained through the cellulose web lying against the drum and accumulated in the compartments under the shell surface of the drum, back to the shell encircling the drum to be once more conducted through the web, takes place by employing a valve consisting of a disk-like member located at one end of the drum and encircling the drum shaft, this member comprising a stationary part and a part rotating along with the drum and lying against it. The compartments collecting washing liquid, located under the shell surface of the drum and which parallel the shaft of the drum and having their bottoms sloping towards the center of the drum, have been connected at their centers through conduits with the rotating part of said valve. The stationary part of the valve, again, is connected through conduits with various point on the shell encircling the drum so that the desired stepwise transfer of the washing liquid in the direction opposite to the direction of rotation of the drum is accomplished.
The valve design of the type described above, which has been used in apparatus of prior art, has turned out in practice to be unsatisfactory. Its most salient drawback is the delay in the transfer of the washing liquid from one step to the other, caused by the required, comparatively long conduits. This delay detracts from the capacity of the apparatus, and it impedes accurate control of the washing process. Moreover, the large quantity of washing liquid carried in the conduits between the drum compartments and the rotary part of the valve causes in connection with the "phase shifts" also mixing of liquid between different steps, whereby the washing result is impaired.